This invention relates to internal combustion engines and more particularly to an engine that uses engine exhaust to heat and reform an alcohol to produce a hydrogen-rich gas reformate that is introduced into the engine.
There has been recent increasing interest in methanol as an alternative fuel for both light and heavy-duty vehicles, particularly in China. This interest has resulted from methanol being the most efficiently and inexpensively produced liquid fuel that can be made from coal and natural gas and also from thermochemical conversion of biomass and waste materials.
In addition to these advantages methanol has a unique set of properties that give it the potential for a substantial increase in efficiency of vehicular engines. One of these properties is its level of knock suppression in spark ignition engines, especially when it is directly injected.
A second property is low temperature endothermic reforming into hydrogen-rich gas, which can then be combusted in the engine. The endothermic reforming can be used in heat recovery from the engine exhaust and the hydrogen-rich gas can be used to further increase efficiency by enabling highly dilute (ultra-lean or heavy EGR) spark ignition operation.
Previous work has been done on taking advantages of these properties. However, it fell substantially short of obtaining the full efficiency increase that can be obtained from methanol-fueled engines.
Extensive literature exists describing thermal decomposition reforming of methanol at low temperature, where energy from the exhaust is used in a substantially endothermic process [J. Agrell, K. Hasselbo, K. Jansson et al., Appl. Catal. A: General 211 (2) (2001) 239; P. Mizsey, E. Newson, T. Truong et al., Appl. Catal. A: General 213 (2) (2001) 233; S. Velu K. Suzuki, M. Okazaki, et al., J. Catal. 194 (2) (2000) 373; Lindstrom B., Pettersson, L. J. Development of a methanol fueled reformer for fuel cell applications, Journal Power Sources 118, 71e8, (2003)]. The thermal decomposition reforming reaction is:CH3OH(l)→2H2+CO; ΔH=+90.7 kJ/mol
The reaction products are referred to as “reformate,” “hydrogen-rich gas” or “syngas”. The nominal heating value of the reformate is around 14% greater than that of methanol.
For ethanol, the thermal decomposition is given byCH3CH2OH(l)→CH4+CO+H2; ΔH=+91.6 kJ/mol
The reaction products in this low temperature ethanol reformation increase the nominal heating value of the ethanol by about 7%.
The amount of work that has been done on ethanol reforming and the use of the reformate in engines is substantially less than the amount of work that has been done on methanol reforming.
Table 1 shows thermodynamic properties of methanol and ethanol near the boiling point, and those of their reformates near the boiling point, to highlight the differences in the reforming process (rather than the phase change). The conditions of the liquid, the gas and the reformate around the boiling point of the liquid are shown, for both 1 bar and 15 bar pressure. The enthalpy of the methanol reformate is about 3000 kJ/Kg higher at both 1and 15 bar) than methanol gas; for ethanol the enthalpy of its reformate is 1000 kJ/kg higher than the ethanol gas. Since the heating value of methanol is about 20 MJ/kg and that of ethanol is about 27 MJ/kg, the methanol enthalpy change in the reformation process is substantial for methanol, but rather small for ethanol.
TABLE 1Thermodynamic properties of methanol and ethanol, and their reformatesTemperaturePressureDensityEnthalpyEntropy(K)(MPa)(kg/m3)(kJ/kg)(kJ/kg K)MethanolLiquidCH3OH3370.1749.0−7.32E+034.39E+00gasCH3OH3380.11.2−6.21E+037.67E+00reformate2H2 + CO3380.10.4−3.34E+031.52E+01LiquidCH3OH4251.5643.5−6.95E+035.28E+00gasCH3OH4301.516.7−6.07E+037.33E+00reformate2H2 + CO4301.54.4−3.09E+031.37E+01EthanolLiquidC2H5OH3500.1737.9−5.88E+033.89E+00gasC2H5OH3600.11.6−5.01E+036.37E+00reformateCH4 + CO + H23600.10.5−3.90E+031.21E+01LiquidC2H5OH4001.5682.6−5.66E+034.42E+00gasC2H5OH4451.522.7−4.86E+036.26E+00reformateCH4 + CO + H24451.56.2−3.71E+031.11E+01
For methanol, the heat of vaporization is about 1000 kJ/kg , while for ethanol it is about 800 kJ/kg.
Another option for exhaust heat recovery from vehicular engines is a Rankine bottoming cycle, similar to that used in electricity generation. In this closed cycle a working fluid (water in electrical power plants) is compressed, heated/vaporized, expanded through a turbine, recooled and recompressed. Organic fluids have been considered for vehicular applications because of advantageous thermodynamic properties, chemical compatibility and stability.
Particular attention has been given to Rankine cycle heat recovery in heavy duty powered vehicles (where diesel engines predominate) because of the great importance of fuel efficiency in applications where there is a large fuel cost per year. However, these Rankine cycles, which did not use methanol or ethanol, captured only a modest fraction of the energy that could be recovered and required a complicated and expensive system to recondense the organic working fluid.
An example of Rankine cycle energy recovery using an organic fluid in a diesel engine vehicle is work reported by Nelson of Cummins Engines, in a DOE funded project. [Chris Nelson, Exhaust Energy Recovery, 2010 Annual Merit Review, Jun. 10, 2010, available at http://www1.eere.energy.gov/vehiclesandfuels/pdfs/merit_review—2010/higheff13 engine_tech/ace 041_nelson—2010_o.pdf]
FIG. 1 shows the initial concept of the prior art Cummins program. There are three loops. In the exhaust loop a fraction of the exhaust from the engine exhaust heat exchanges with the organic fluid in the WHR (Waste Heat Recovery) unit. The fraction is controlled by the EGR valve. The cooler exhaust is then combined with the air, and then cooled further by the cooler organic fluid. Combined EGR/air is then introduced into the engine.
The organic fluid is compressed and then heated in the CCAC (Combined Charge Air Cooler) heat exchanger. The organic fluid is then further heated by the exhaust in the het exchanger marked EGR WHR. The organic fluid is then expanded in the turbine, cooled/condensed by water/engine coolant, and then recompressed.
Finally, the last circuit is the engine coolant loop. The fraction of the energy from the exhaust not recovered in the turbine is then disposed off in a liquid-air heat exchanger (i.e., radiator).
Nelson initially predicted a 10% Brake Thermal Efficiency (BTE) benefit, using R245fa as an organic working fluid. However, the Cummins program had problems in the CCAC with condensation and corrosion, as the material chosen for the heat exchanger was aluminum. The effort changed focus because of these difficulties, and moved to provide energy recovery during off-peak operating conditions, with about 5% waste energy recovery from the exhaust.
It should be noted that in addition to the issues with the heat exchanger, there was a substantial increase in heat rejection complexity, as a condenser cooler was needed to remove in a radiator the bulk of the power that was recovered from the exhaust. Indeed, assuming an efficiency of 30% for the Rankine cycle, 70% of the energy recovered from the exhaust needs to be removed through a radiator instead of just being released by the exhaust.